1. Field of the Invention
The invention relates generally to the fields of medicine and cell biology and, more specifically, to the fields of infectious disease and regulation of apoptosis.
2. Background Information
Apoptosis, or programmed cell death, is an important process in the development and homeostasis of multicellular organisms. Additionally, apoptosis of infected cells provides the host organism with an effective defense mechanism against pathogens. Alterations in the normal process of apoptosis occur in various pathological conditions, including cancer, autoimmune diseases, degenerative syndromes and infectious diseases.
Some of the proteins involved in apoptosis have been identified, and associations among these proteins described. The principal effectors of apoptosis are a family of intracellular proteases known as caspases. Initiation of the caspase proteolytic cascade requires assembly of caspase precursors on an adaptor protein. Members of a family of proteins known as Bcl-2 proteins are key regulators of apoptosis, in part because they determine whether this adaptor/procaspase complex can form. Bcl-2 family members also regulate apoptosis by regulating mitochondrial integrity, thereby controlling release of the caspase co-activator protein cytochrome c. Bcl-2 family members have been found in organisms as diverse as mammals, nematodes, fruitflies and viruses. However, no Bcl-2 family members have been identified to date in prokaryotes.
Dysregulation of host cell apoptosis figures prominently in the pathophysiology of many bacterial infectious diseases. Some of the effects of these bacteria on the host cell apoptotic machinery have been partially elucidated for certain bacteria. However, the bacterial proteins that exert these effects remain to be identified and their mechanisms of action characterized.
A number of intracellular bacteria act by inducing host cell apoptosis, often in a cell-type specific manner. For example, Shigella flexneri, a causative agent of bacillary dysentery, infects macrophages and induces their apoptosis in part via activation of caspase-1. Concomitant with apoptosis is a release of inflammatory cytokines that cause migration of polymorphonuclear leukocytes across the intestinal epithelium to the site of the infection, which compromises the integrity of the epithelial barrier, promoting massive secondary invasion of the bacteria and acute inflammation. Other examples of pro-apoptotic infectious bacteria include Salmonella, Listeria, Legionella, Yersinia and Coxiella, which target various regulatory molecules in the host cell apoptotic pathway.
Certain infectious bacteria both induce and inhibit host cell apoptosis. For example, Mycobacterium tuberculosis, a causative agent of tuberculosis, induces apoptosis in macrophages in part by down-regulation of expression of the host anti-apoptotic protein Bcl-2. Paradoxically, M. tuberculosis infection also protects cells against apoptosis in part via induction of the NF-κB cell survival pathway, and also by enhancing production of soluble TNF receptor 2, which neutralizes the pro-apoptotic activity of TNFα. Both pro- and anti-apoptotic activities, which are possibly manifested during different stages of infection, may be needed for establishment of a persistent infection. Other bacteria that both induce and inhibit host cell apoptosis by various mechanisms include Chlamydia and Rickettsia. 
Although apoptosis is generally considered to be a eukaryotic process, many bacteria undergo an apoptotic-like process that prevents multiplication under conditions of environmental stress. This adaptive response is particularly apparent in prokaryotic organisms that display developmental programs, such as sporulation in Streptomyces and Bacillus and the formation of nonculturable but viable cells in various Gram-negative bacteria. Similarities between eukaryotic apoptosis and the prokaryotic apoptotic-like process include induction of protein synthesis, proteolytic activity, DNA fragmentation, RNA degradation and cell shrinkage.
The development of drug resistant strains of bacteria is a serious health concern. It is currently estimated that within the next 10 years, virtually all antibiotics currently employed for treating bacterial infections will no longer be effective, due to microbial resistance. New therapeutic agents are urgently needed to meet the threat of drug-resistant bacteria. In view of the important role of apoptosis in the bacterial life cycle and in pathogenesis, there exists a need to identify bacterial molecules that regulate bacterial and host cell apoptosis. Such molecules can be used in the development of novel antibiotics, as well as in the development of therapeutic agents for the treatment of other disorders of apoptotic regulation. The present invention satisfies this need and provides related advantages as well.